Method and apparatus for frame rate determination without decoding in a spread spectrum receiver

ABSTRACT

A spread spectrum receiver ( 124 ) for receiving and decoding a data frame having one of a plurality of coding rates is coupled to a rate determination device ( 132 ). The rate determination device ( 132 ) is coupled to receive a data frame coded at one of the plurality of coding rates and to receive symbol data from the receiver ( 124 ). The rate determination device ( 132 ) is adapted to determine a probability based at least upon an rate-based symbol repetition constraints within the data frame, wherein the probability is an indication of the rate at which the data frame is encoded.

TECHNICAL FIELD

[0001] This patent relates to receivers for use in a spread spectrumcommunication system.

BACKGROUND

[0002] In a spread spectrum communication system, downlink transmissionsfrom a base station to a mobile station include a pilot channel and aplurality of traffic channels. The pilot channel is demodulated by allusers. Each traffic channel is intended for demodulation by a singleuser, though more than one channel may be intended for a given user.Therefore, each traffic channel is spread using a unique code known byboth the base station and the mobile station. The pilot channel isspread using a code known by the base station and all mobile stations.Multiplication of the pilot channel and traffic channel symbols byunique code sequences comprised of chips having duration much less thanthe symbol duration spreads the spectrum of transmissions in the system.

[0003] One example of a spread spectrum communication system is acellular radiotelephone system according to Telecommunications IndustryAssociation/Electronic Industry Association (TIA/EIA) Interim StandardIS-95, “Mobile Station-Base Station Compatibility Standard for Dual-ModeWideband Spread Spectrum Cellular System” (IS-95). Individual users inthe system use the same frequency spectrum but are distinguishable fromeach other through the use of individual spreading codes. IS-95 is anexample of a direct sequence code division multiple access (DS-CDMA)communication system. In a DS-CDMA system, transmissions are spread by apseudorandom noise (PN) code. Data is spread by a sequence of chips,where the chip is the spread spectrum minimal-duration keying element.

[0004] Other spread spectrum systems include radiotelephone and datasystems operating at various frequencies and utilizing various spreadingtechniques. Among these additional systems are third-generation spreadspectrum communication systems (3G), wideband code division multipleaccess systems (W-CDMA) and CDMA2000.

[0005] As allowed in IS-95 and the other direct sequence spread spectrumcommunication protocols, the forward link frames can be at one of fourpossible rates: full-rate, half-rate, quarter-rate and eighth-rate. Inexisting receivers, a rate determination algorithm is used to determinethat rate at which the forward link frames are encoded. As is generallypracticed, the received frames are decoded at each of the four possiblerates using a Viterbi decoder. Typically, the inputs to the ratedetermination algorithm include the cyclical redundancy check (CRC) dataappended to the full and half-rate frames, estimates of the channelsymbol error rates based on re-encoding the Viterbi decoder output,quality bits associated with the distance between merging paths in thedecoder trellis, and the total Euclidean distance between the output ofthe receiver and the decoded codeword.

[0006] This process is inefficient. It suffers from added current drainassociated with the multiple decodings and may require a faster clockand additional circuitry. The frame needs only to be decoded once toobtain the coded data, but at the correct rate. Decoding at the wrongrate results in unusable data. Various approaches have been suggested topredict the frame rate prior to decoding, but in most instances theseapproaches are computationally complex and are not sufficientlyreliable.

[0007] Thus, there is a need for a spread spectrum receiver thatprovides-pre-decoding rate determination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] This disclosure will describe several embodiments to illustrateits broad teachings. Reference is also made to the attached drawings.

[0009]FIG. 1 is a block diagram of a communication system.

[0010]FIG. 2 is a block diagram of a spread spectrum receiverincorporating rate determination.

[0011]FIG. 3 is a flow diagram illustrating a method of ratedetermination in a spread spectrum receiver.

DETAILED DESCRIPTION

[0012] A spread spectrum receiver incorporates rate determination. In anembodiment of the receiver, a maximum-likelihood decision rule isapplied for pre-decoding rate determination. While the followingdiscussion refers to the IS-95 interim standard, the disclosure hasapplication to virtually any spread spectrum communication system.

[0013] Referring to FIG. 1, a communication system 100 includes aplurality of base stations including base station 102 and base station104. Each base station is separately coupled to a mobile switchingcenter 106, which controls communication within the system and betweenthe system and the public switch telephone network 108. Thecommunication system 100 may be a cellular telephone system operatingaccording to IS-95, CDMA2000, 3G, W-CDMA or other direct sequence spreadspectrum communication standards, another type of cellular or mobilecommunication system, a fixed wireless loop system or other type ofradio system.

[0014] Each base station is configured for radio frequency (RF)communication with fixed or mobile transceivers such as mobile station110. Accordingly, each base station includes a receiver such as receiver112 of the base station 102 and receiver 114 of the base station 104 anda transmitter such as transmitter 116 of the base station 102 and thetransmitter 118 of the base station 104. Each transmitter transmits aspread spectrum signal including a first signal and a second signal, thefirst signal being substantially orthogonal to the second signal. Thefirst signal may be, for example, the pilot channel in the IS-95implementation and the second signal may be one or more trafficchannels. In IS-95, the pilot channel and the traffic channels arecovered using a Walsh or Hadamard code, so that at transmission, thechannels are all substantially orthogonal.

[0015] The mobile station 110 includes a RF front end 120, a receiver124, a transmitter 126, a control section 128 and a user interface 130.The RF front end 120 filters the spread spectrum signals and providesconversion to baseband signals. The RF front end 120 further providesanalog to digital conversion, converting the baseband signals to streamsof digital data for further processing. The receiver 124 demodulates thedigital data and provides the demodulated data to the control section128.

[0016] The control section 128 controls overall operation of the mobilestation 110, including assignment of the RAKE fingers. The controlsection also controls interaction of the radio components and the userinterface 130. The user interface 130 typically includes a display, akeypad, a speaker and a microphone. The transmitter 126 modulates datafor transmission to a remote receiver, such as one of the base stations.The modulated data are processed by the front end 120 and transmitted atradio frequency.

[0017] Positioned between the receiver 124 and the RF front end 120 is arate determination device 132. The rate determination device 132 mayoperate under the control of control 128 or may contain its ownprocessor or other suitable processing and control capability (notdepicted) to function as described herein and determines the rate of theincoming frames and communicates this information to the receiver 124.The receiver 124 uses the rate information to decode the received framesaccording to the rate information in an efficient manner. That is, therate determination device 132 receives data from the receiver 124 thatis then used to predict, a priori, the rate of the next frame to bedecoded. The determined rate information is then communicated by therate determination device 132 back to the receiver 124 thus allowing thereceiver 124 to decode the frame one time at the correct rate.

[0018] The rate determination device 132 may use a single ratedetermination algorithm or multiple rate determination algorithmstailored to the various conditions in which the mobile station 110 isoperating. For example, a rate determination algorithm may haveparameters that are a function of the signal-to-noise ratio (SNR), thefading rate of the channel, or other channel characteristics or thealgorithm may be specifically tailored for a particular coding scheme,such as Rate Set 1 or Rate Set 2. The rate determination device 132 maybe informed of the channel conditions by the receiver 124 or by thecontrol 128 in order to select an appropriate rate determinationalgorithm. Alternatively the rate determination device 132 mayself-determine the channel characteristics.

[0019]FIG. 2 illustrates a RAKE receiver 200 that may be incorporatedinto the receiver 124. The RAKE receiver 200 includes a plurality offingers, generally illustrated as finger 202. Each finger 202 contains adespreader 204 for despreading the baseband signals received from the RFfront end 120. The output signals of each finger are then multiplied bythe conjugate of the estimate of the complex channel gain for the givenfinger and combined in a combiner 208 to provide the data signalestimate, s(t). The rate information is provided by the ratedetermination device 132 to the decoder 206.

[0020] The rate determination device 132 may use any rate determinationalgorithm suitable for the communication protocol being used within thecommunication system 100 and for the existing channel conditions. As anexample, described herein are rate determination algorithms suitable foruse with a spread spectrum code division multiple access communicationsystem. While the above-described process of decoding each frame at eachof the four possible rates may be used as one of the possible ratedetermination algorithms, it is preferable that the rate determinationalgorithm provides an indication of frame rate prior to decoding toimprove computational efficiency as well as reduce decoding delay. Inthat regard, additional algorithms are described herein in connectionwith the IS-95 interim standard.

[0021] Two rate sets are defined for IS-95. In the first, referred to asRate Set 1, a full-rate frame contains 192 information symbols that areencoded into 384 code symbols by a rate {fraction (1/2)} convolutionalencoder. Half-rate, quarter-rate and eighth-rate frames contain 96, 48and 24 information symbols and 192, 96, and 48 code symbols,respectively. Symbol repetition is used to fill up the frame, so thathalf-rate code symbols are repeated twice, and quarter-rate andeighth-rate code symbols are repeated four and eight times,respectively. Interleaving is used to mitigate the degradation of thecommunication link caused by fading.

[0022] In Rate Set 2, a full-rate frame contains 288 informationsymbols, while half-rate, quarter-rate and eighth-rate frames contain144, 72, and 36 information symbols, respectively. Rate Set 2 frames areencoded with the same rate {fraction (1/2)} convolutional code used forRate Set 1. Symbols of the half-rate, quarter-rate, and eighth-rateframes are repeated 2, 4 and 8 times, respectively, resulting in a totalof 576 code symbols per frame, regardless of rate. After repetition, twoof every six code symbols are punctured to reduce the number of symbolsper frame to 384.

[0023] To simplify the discussion of the immediately following ratedetermination algorithm, only Rate Set 1 frames are considered. However,one of ordinary skill in the art will appreciate the modificationsrequired to adapt the algorithm to Rate Set 2. Because of the repetitionscheme used for Rate Set 1, it is possible to consider blocks of eightbinary symbols. If the frame is full rate, no constraints are imposed onthe sequence of symbols {b₁, b₂, . . . , b₈}. However, for half-rateframes, a sequence is allowed if and only if b₁=b₂, b₂=b₄, b₅=b₆ andb₇=b₈. Quarter-rate frames require both that b₁=b₂=b₃=b₄ andb₅=b₆=b₇=b₈. Eighth rate frames admit only the two binary sequences forwhich b₁=b₂= . . . =b₈. Let the sequence {y₁, y₂, . . . , y₈} denote thecorresponding output of the combiner of the RAKE receiver, e.g.,combiner 208.

[0024] From the above description of the combiner output, even in thepresence of fading, the ratio of the mean to the variance is the samefor all symbols within a given frame, providing that forward link powercontrol is slow so that the traffic-to-pilot power ratio is constantover the frame. If fast power control is used so that the traffic powerchanges every power control group, the weighting scheme must be modifiedso that the ratio of the symbol mean and variance is held constant overthe frame.

[0025] If no a priori distribution is assigned to the possible framerates, then the maximum a posteriori decision (MAP) rule is equivalentto the maximum-likelihood (ML) decision rule which selects the rater_(i), i≡{1, 2, 3, 4}, for which:

log(p({y ₁ }|r _(i))/p({y ₁ }|r ₁))

[0026] is maximized. From this equation it is apparent that only threelog-likelihood ratios need to be calculated, since the log-likelihoodratio for full rate will always be 0. The four log-likelihood ratiosare: $\begin{matrix}{{\log \frac{p( \{ y_{i} \} \middle| r_{1} )}{p( \{ y_{i} \} \middle| r_{1} )}} = 0} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{2} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{4\quad \log \quad 2} - {2y\quad \Delta_{1}}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{3} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{6\quad \log \quad 2} - {2y\quad ( {\Delta_{1} + \Delta_{2}} )}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{4} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{7\quad \log \quad 2} - {2y\quad ( {\Delta_{1} + \Delta_{2} + \Delta_{3}} )}}}\end{matrix}$

[0027] The terms Δ₁, Δ₂ and Δ₃ are: $\begin{matrix}{\Delta_{1} = {\sum\limits_{i = 1}^{4}\quad {\delta ( {y_{{2i} - 1} \cdot y_{2i}} )}}} \\{\Delta_{2} = {\sum\limits_{i = 0}^{1}\quad {\delta ( {s_{{4i} + {1.4i} + 2} \cdot s_{{4i} + {3.4i} + 4}} )}}} \\{\Delta_{3} = {\delta ( {s_{1,4} \cdot s_{5.8}} )}}\end{matrix}$

 Δ₃=δ(s_(1.4) ·s_(5.8))

[0028] where the partial-sum s_(j,k) is given by$s_{j.k} = {\sum\limits_{i = j}^{k}\quad {y_{i}.}}$

[0029] The parameter γ is the ratio of the mean and variance of y_(i),the output of the combiner for each symbol. This mean, however, is notknown a priori. By modifying the receiver hardware, an estimation of γcan be made according to the equation:$\hat{\gamma} = {\frac{\sum\limits_{i = 1}^{384}\quad {y_{i}}}{( {A_{p}^{2}{\sum\limits_{i = 1}^{384}\quad {\sum\limits_{j = 1}^{3}\quad {{\hat{\alpha}}_{i \cdot j}}^{2}}}} )\sigma^{2}} \approx \frac{A_{t}{\mu ( r_{i} )}}{A_{p}\sigma^{2}}}$

[0030] where A_(p) ² is the pilot amplitude and a_(i,j) is the complexchannel gain of the different multipath rays. In the middle expression,the first term in the denominator is the sum of the squares of thechannel estimates for the entire frame, and the second term σ² is knownsince it is determined by the receiver automatic gain control (AGC)circuit. Thus, while relatively computationally complex, it is possibleto make an estimate of the parameter γ.

[0031] All of the data dependence in the log-likelihood ratios iscaptured in the terms Δ1, Δ2 and Δ3. Examination of these terms yieldsthe following observations:

[0032] i) if two adjacent symbols (or partial sums) have the same sign,the corresponding contribution to the log-likelihood ratio is 0;

[0033] ii) the contribution of adjacent symbols (partial sums, s_(i,j))different in sign to the log-likelihood ratio is equal to the minimumamplitude of the two symbols (partial sums);

[0034] iii) symbols (partial sums) differing in sign are attributedsignificance equal to γ, the ratio of the symbol mean and variance forthe given frame.

[0035] The first observation is consistent with intuition since it isonly when adjacent symbols have different signs that they are notrepetitions of the same symbol. Symbols that agree yield essentially noinformation because even if the rate is such that the second symbol isnot required to be a repetition of the first, it is still allowed to bethe same binary value as the first. The second observation also agreeswith intuition since the confidence that two symbols agree or differ insign should correspond (approximately) to the smaller of the two symbolamplitudes (e.g., given two symbols with amplitudes 0.25 and 100, theconfidence that the two symbols have the same or different sign shouldbe approximately equal to the smaller of the two amplitudes; i.e., 0.25.

[0036] In implementation, the rate determination device 132 implementsthe above-described log-likelihood ratios. The rate decision is based onwhich of the log-likelihood ratios is largest.

[0037] An alternative, simplified rule can be constructed for selectingbetween the four rates based on the above observations. Essentially, themeasure of interest is the sum reliability of those pairs of symbols (orpartials sums$ {s_{j.k} = {\sum\limits_{i = j}^{k}\quad y_{i}}} )$

[0038] having opposite sign relative to the sum reliability of all pairsof symbols (or partial sums) in the frame. Thus, the sum reliability ofall pairs of symbols (partial sums) is used to normalize the reliabilitypairs of the symbols (partial sums) having opposite sign. With thismotivation, the following three measures may be defined: $\begin{matrix}{\Psi_{2} = \frac{\sum\limits_{i = 0}^{191}\quad {\delta ( {y_{{2i} + 1},y_{{2i} + 2}} )}}{\sum\limits_{i = 0}^{191}\quad {\min \{ {{y_{{2i} + 1}},{y_{{2i} + 2}}} \}}}} \\{\Psi_{3} = \frac{\sum\limits_{i = 0}^{95}\quad {\delta ( {s_{{4i} + {1.4i} + 2},s_{{4i} + {3.4i} + 4}} )}}{\sum\limits_{i = 0}^{95}\quad {\min \{ {{s_{{4i} + {1.4i} + 2}},{s_{{4i} + {3.4i} + 4}}} \}}}} \\{\Psi_{4} = \frac{\sum\limits_{i = 0}^{47}\quad {\delta ( {s_{{8i} + {1.8i} + 4},s_{{8i} + {5.8i} + 8}} )}}{\sum\limits_{i = 0}^{47}\quad {\min \{ {{s_{{8i} + {1.8i} + 4}},{s_{{8i} + {5.8i} + 8}}} \}}}}\end{matrix}$

[0039] In the above, all three summations have been extended over theentire frame.

[0040] All three measures defined above lie in the interval [0,1]. Themeasure ψ₂ weighs the reliability of pairs of symbols having oppositesign against the sum reliability of all pairs of symbols in the frame.Only pairs of symbols having opposite sign contribute to the numeratorof ψ₂, while all pairs of symbols contribute to the denominator.Similarly, ψ₃ and ψ₄ measure the reliability of pairs of partial sumshaving opposite sign relative to the reliability of all pairs of partialsums in the frame. These expressions are relatively computationallyequivalent.

[0041] The measures have the following properties:

[0042] i) for any value of the signal-to-noise ratio E_(b)/N₀,

[0043] E(Ψ₂|r₁)=0.5

[0044] E(Ψ₃|r₁)=E(Ψ₃|r₂)=0.5

[0045] E(Ψ₄|r₁)=E(Ψ₄|r₂)=E(Ψ₄|r₃)=0.5

[0046] ii) in the limit as Eb/No →∞,

[0047] E(Ψ₂|r₂)=E(Ψ₂|r₃)=E(Ψ₂|r₄)=0

[0048] E(Ψ₃|r₃)=E(Ψ₃|r₄)=0

[0049] E(Ψ₄|r₄)=0.

[0050] The Rate Set 1 decision rule is thus:

[0051] Given a threshold η₁,

[0052] i) if Ψ₄≦η₁, rate=r₄;

[0053] ii) if Ψ₃≦η₁ and Ψ₄>η₁, rate=r₃;

[0054] iii) if Ψ₂≦η₁ and Ψ₃>η₁ and Ψ₄>η₁, rate=r₂;

[0055] iv) if Ψ₂>η₁ and Ψ₃>η₁ and Ψ₄>η₁, rate=r₁

[0056] This decision rule exhibits an error floor that is a function ofthe threshold. The error floor may be made arbitrarily small by reducingthe threshold η₁ (the limit on the error floor is 2 ⁻⁴⁸, which is theprobability that all 48 symbol pairs of a quarter-rate frame are equal).However, as the threshold is decreased, rate detection performance maybe degraded at low values of E_(b)/N₀. Thus, reducing the threshold η₁reduces the error floor and improves asymptotic performance whiledegrading rate detection performance at low signal-to-noise ratios.Ideally, if an estimate of the channel quality could be obtained, thethreshold η₁ would be based on this quality estimate. This decision rulealso suggests better performance on a fading channel than on a staticchannel.

[0057] For Rate Set 2, a separate set of metrics is required. Thesemetrics are: $\begin{matrix}{\Omega_{2} = \frac{\sum\limits_{i = 0}^{95}\quad {\delta ( {y_{{4i} + 1},y_{{4i} + 2}} )}}{\sum\limits_{i = 0}^{95}\quad {\min \{ {{y_{{4i} + 1}},{y_{{4i} + 2}}} \}}}} \\{\Omega_{3} = \frac{\sum\limits_{i = 0}^{47}\quad \lbrack {{\delta ( {s_{{8i} + {1.8i} + 2},y_{{8i} + 3}} )} + {\delta ( {y_{{8i} + 4},s_{{8i} + {5.8i} + 6}} )} + {\delta ( {y_{{8i} + 7},y_{{8i} + 8}} )}} \rbrack}{\sum\limits_{i = 0}^{47}\quad \lbrack {{\min ( {{s_{{8i} + {1.8i} + 2}},{y_{{8i} + 3}}} )} + {\min ( {{y_{{8i} + 4}},{s_{{8i} + {5.8i} + 6}}} )} + {\min ( {{y_{{8i} + 7}},{s_{{8i} + 8}}} )}} \rbrack}} \\{\Omega_{4} = \frac{\sum\limits_{i = 0}^{23}\quad \lbrack {{\delta ( {s_{{16i} + {1.16i} + 3},s_{{16i} + {4.16i} + 6}} )} + {\delta ( {s_{{16i} + {7.16i} + 8},s_{{16i} + {9.16i} + 11}} )} + {\delta ( {s_{{16i} + {12.16i} + 14},s_{{16i} + {15.16i} + 16}} )}} \rbrack}{\sum\limits_{i = 0}^{23}\quad \lbrack {{\min ( {{s_{{16i} + {1.16i} + 3}},{s_{{16i} + {4.16i} + 6}}} )} + {\min ( {{s_{{16i} + {7.16i} + 8}},{s_{{16i} + {9.16i} + 11}}} )} + {\min ( {{s_{{16i} + {12.16i} + 14}},{s_{{16i} + {15.16i} + 16}}} )}} \rbrack}}\end{matrix}$

[0058] The difference between these definitions and those given abovefor Rate Set 1 are due to the difference in the symbol repetitionpatterns used for the two rate sets. The properties ascribed above tothe measures {ψ₂, ψ₃ and ψ₄} apply also to the measures {Ω₂, Ω₃, Ω₄}.The Rate Set 2 decision rule is given by the following:

[0059] Given a threshold η₂,

[0060] i) if Ω₄≦η₂, rate=r₄;

[0061] ii) if Ω₃≦η₂ and Ω₄>η₂, rate=r₃;

[0062] iii) if Ω₂≦η₂ and Ω₃>η₂ and Ω₄>η₂, rate=r₂;

[0063] iv) if Ω₂>η₂ and Ω₃>η₂ and Ω₄>η₂, rate=r₁

[0064] This rate detection rule has little impact on the decoded frameerror rate if the target frame error probability is 10⁻² or higher.Advantageously, (with respect to rate determination only) because of therelatively weaker coding scheme of Rate Set 2, a greater value ofE_(b)/N₀ is required to achieve the required 10⁻² frame error rateperformance. Furthermore, because of the higher coding rate, a givenvalue of the bit signal-to-noise ration E_(b)/N_(o) corresponds to alarger value of the symbol signal-to-noie ratio E_(s)/N_(o) for Rate Set2 than for Rate Set 1. Since it is E_(s)/N_(o), rather than E_(b)/N_(o),which most directly affects rate detection performance, overall ratedetection performance is slightly better for Rate Set 2 than for RateSet 1.

[0065] An additional decision rule that can be used for Rate Set 1, andcan be extended to Rate Set 2 as well as to other rate set definitionswith multiple frame rates implemented using repetition and puncturing,is given by the following:

[0066] Rate=r_(i), where

[0067] i=arg max {Λ_(i) ³, iε{1, 2, 3, 4}},

[0068] with${\Lambda_{1}^{3} = {a_{1}( {( {\sum\limits_{i = 1}^{8}\quad y_{i}^{2}} ) - {8\quad \delta^{2}}} )}},$

[0069] Λ₂ ³=a₂(s_(1,2) ²+s_(3,4) ²+s_(5,6) ²+s_(7,8) ²)−8δ²),

[0070] Λ₃ ³=a₃((s_(1,4) ²+s_(5,8) ²)−8δ²),

[0071] Λ₄ ³=a₄(s_(1,8) ²−8δ²),

[0072] and δis the interference variance at the output of the RAKEcombiner, which can be estimated as${{\hat{\delta}}^{2} = {( {\sum\limits_{i = 0}^{383}\quad {y_{i}}^{2}} ) - ( {\sum\limits_{i = 0}^{383}\quad {y_{i}}} )^{2}}},$

[0073] or over a smaller subset of the channel symbols.

[0074] A maximin criteria can be used to select {a_(i)}. However,simulations have shown that the following values for {a_(i)} yieldbetter results,${a_{1} = 1},{a_{2} = \frac{1}{\sqrt{2}}},{a_{3} = \frac{1}{2}},{a_{4} = {\frac{1}{2\sqrt{2}}.}}$

[0075] so that this additional Rate Set 1 decision rule is given by

[0076] Rate=r_(i), where

[0077] i=arg max {Λ_(i) ³, iε{l, 2, 3, 4}},

[0078] with $\begin{matrix}{{\Lambda_{1}^{3} = ( {( {\sum\limits_{i = 1}^{8}\quad y_{i}^{2}} ) - {8\quad \delta^{2}}} )},} \\{{\Lambda_{2}^{3} = {\frac{1}{\sqrt{2}}( {( {s_{1,2}^{2} + s_{3,4}^{2} + s_{5,6}^{2} + s_{7,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{3}^{3} = {\frac{1}{2}( {( {s_{1,4}^{2} + s_{5,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{4}^{3} = {\frac{1}{2\sqrt{2}}( {s_{1,8}^{2} - {8\quad \delta^{2}}} )}},}\end{matrix}$

[0079] This patent describes several specific embodiments includinghardware and software embodiments of apparatus and methods forpre-decoding rate determination. However, one of ordinary skill in theart will appreciate that various modifications and changes can be madeto these embodiments. Accordingly, the specification and drawings are tobe regarded in an illustrative rather than restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent patent.

We claim:
 1. A spread spectrum receiver for receiving and decoding adata frame having one of a plurality of coding rates, the spreadspectrum receiver comprising: a rate determination device coupled toreceive a data frame coded at one of the plurality of coding rates andto receive symbol data from the receiver, the rate determination devicebeing adapted to determine a probability based at least upon rate-basedsymbol repetition constraints within the data frame, wherein theprobability is an indication of the rate at which the data frame isencoded.
 2. The spread spectrum receiver of claim 1, wherein theprobability comprises a log likelihood ratio based at least upon therate-based symbol repetition constraints within the data frame.
 3. Thespread spectrum receiver of claim 2, wherein the log likelihood ratioindicative of the rate is the maximum of the group of log likelihoodratios comprising: $\begin{matrix}{{\log \frac{p( \{ y_{i} \} \middle| r_{1} )}{p( \{ y_{i} \} \middle| r_{1} )}} = 0} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{2} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{4\quad \log \quad 2} - {2y\quad \Delta_{1}}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{3} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{6\quad \log \quad 2} - {2y\quad ( {\Delta_{1} + \Delta_{2}} )}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{4} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{7\quad \log \quad 2} - {2y\quad {( {\Delta_{1} + \Delta_{2} + \Delta_{3}} ).}}}}\end{matrix}$


4. The spread spectrum receiver of claim 1, comprising a decision rulebased upon the probability.
 5. The spread spectrum receiver of claim 4,wherein the decision rule comprises: i) if Ψ₄≦η₁, rate=r₄; ii) if Ψ₃≦η₁and Ψ₄>η₁, rate=r₃; iii) if Ψ₂≦η₁ and Ψ₃>η₁ and Ψ₄>η₁, rate=r₂; iv) ifΨ₂>η₁ and Ψ₃>η₁ and Ψ₄>η₁, rate=r₁.
 6. The spread spectrum receiver ofclaim 4, wherein the decision rule comprises: i) if Ω₄≦η₂, rate=r₄; ii)if Ω₃≦η₂ and Ω₄>η₂, rate=r₃; iii) if Ω₂≦η₂ and Ω₃>η₂ and Ω₄>η₂, rate=r₂;iv) if Ω₂>η₂ and Ω₃>η₂ and Ω₄>η₂, rate=r₁.
 7. The spread spectrumreceiver of claim 4, wherein the decision rule comprises: Rate=r_(i),where i=arg max {Λ_(i) ³, iε{1, 2, 3, 4}}, with $\begin{matrix}{{\Lambda_{1}^{3} = ( {( {\sum\limits_{i = 1}^{8}\quad y_{i}^{2}} ) - {8\quad \delta^{2}}} )},} \\{{\Lambda_{2}^{3} = {\frac{1}{\sqrt{2}}( {( {s_{1,2}^{2} + s_{3,4}^{2} + s_{5,6}^{2} + s_{7,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{3}^{3} = {\frac{1}{2}( {( {s_{1,4}^{2} + s_{5,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{4}^{3} = {\frac{1}{2\sqrt{2}}( {s_{1,8}^{2} - {8\quad \delta^{2}}} )}},}\end{matrix}$

and δ is at least an estimate of the interference variance at the outputof the RAKE combiner.
 8. The spread spectrum receiver of claim 1,comprising a plurality of rate determination algorithms, the ratedetermination device operable to select one of the plurality of ratedetermination algorithms based upon a channel characteristic.
 9. Thespread spectrum receiver of claim 8, wherein the channel characteristiccomprises one of the group of characteristics comprising:signal-to-noise ratio (SNR), symbol energy-to-to-noise ratio (E_(s)/N₀),bit energy-to-noise ratio (E_(b)/N₀), characteristics of the channelfading process such as fading rate, and rate set.
 10. In a spreadspectrum receiver for receiving and decoding a data frame having one ofa plurality of coding rates, a method for decoding received data framescomprising: determining a probability based upon rate-based symbolrepetition constraints within a received data frame; determining a framerate based upon the probability; and decoding the received data frameaccording to the frame rate.
 11. The method of claim 10, wherein thestep of determining a probability comprises determining a log likelihoodratio based at least upon the rate-based symbol repetition constraintswithin the data frame.
 12. The method of claim 11, wherein the loglikelihood ratio indicative of the rate is the maximum of the group oflog likelihood ratios comprising: $\begin{matrix}{{\log \frac{p( \{ y_{i} \} \middle| r_{1} )}{p( \{ y_{i} \} \middle| r_{1} )}} = 0} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{2} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{4\quad \log \quad 2} - {2y\quad \Delta_{1}}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{3} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{6\quad \log \quad 2} - {2y\quad ( {\Delta_{1} + \Delta_{2}} )}}} \\{{\log \frac{p( \{ y_{i} \} \middle| r_{4} )}{p( \{ y_{i} \} \middle| r_{1} )}} \approx {{7\quad \log \quad 2} - {2y\quad {( {\Delta_{1} + \Delta_{2} + \Delta_{3}} ).}}}}\end{matrix}$


13. The method of claim 10, wherein the step of determining the framerate comprises applying a decision rule based upon the probability. 14.The method of claim 13, wherein the decision rule comprises: i) ifΨ₄≦η₁, rate=r₄; ii) if Ψ_(3≦η) ₁ and Ψ₄>η₁, rate=r₃; iii) if Ψ₂≦η₁ andΨ₃>η₁ and Ψ₄>η₁, rate=r₂; iv) if Ψ₂>η₁ and Ψ₃>η₁ and Ψ₄>η₁, rate=r₁. 15.The method of claim 13, wherein the decision rule comprises: i) ifΩ₄≦η₂, rate=r₄; ii) if Ω_(3≦η) ₂ and Ω₄>η₂, rate=r₃; iii) if Ω₂≦η₂ andΩ₃>η₂ and Ω₄>η₂, rate=r₂; iv) if Ω₂>η₂ and Ω₃>η₂ and Ω₄>η₂, rate=r₁. 16.The spread spectrum receiver of claim 13, wherein the decision rulecomprises: rate=r_(i), where i=arg max {Λ_(i) ³, iε{1, 2, 3, 4}}, with$\begin{matrix}{{\Lambda_{1}^{3} = ( {( {\sum\limits_{i = 1}^{8}\quad y_{i}^{2}} ) - {8\quad \delta^{2}}} )},} \\{{\Lambda_{2}^{3} = {\frac{1}{\sqrt{2}}( {( {s_{1,2}^{2} + s_{3,4}^{2} + s_{5,6}^{2} + s_{7,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{3}^{3} = {\frac{1}{2}( {( {s_{1,4}^{2} + s_{5,8}^{2}} ) - {8\quad \delta^{2}}} )}},} \\{{\Lambda_{4}^{3} = {\frac{1}{2\sqrt{2}}( {s_{1,8}^{2} - {8\quad \delta^{2}}} )}},}\end{matrix}$

and δ is at least an estimate interference variance at the output of theRAKE combiner.
 17. The method of claim 10, comprising the step ofselecting from a plurality of rate determination algorithms a ratedetermination algorithm to be used to determine the frame rate basedupon a channel characteristic.
 18. The method of claim 17, wherein thechannel characteristic comprises one of the group of characteristicscomprising: signal-to-noise ratio (SNR), symbol energy-to-noise ratio(E_(s)/N₀), bit energy-to-noise ratio (E_(b)/N₀), characteristics of thechannel fading process such as fading rate, and rate set.
 19. A spreadspectrum receiver for receiving and decoding a data frame having one ofa plurality of coding rates comprising: means for determining aprobability based upon rate-base symbol repetition constraints within areceived data frame; means for determining a frame rate based upon theprobability; and means for decoding the received data frame according tothe frame rate.
 20. The spread spectrum receiver of claim 19, comprisingmeans for selecting from a plurality of rate determination algorithms arate determination algorithm to be used to determine the frame ratebased upon a channel characteristic.